1. Field of Invention
The present invention relates to an overvoltage protection (OVP) circuit for use in a charger circuit system and a charger circuit with OVP function.
2. Description of Related Art
Please refer to FIG. 1, which shows a prior art charger circuit. In this circuit, the OVP control circuit 11 detects whether the voltage supply VH exceeds a threshold value before charging the battery 14 (this battery for example is a battery in a portable electronic device); the OVP control circuit 11 turns on the transistor switch M1 only when the voltage supply VH is within a safety range, so that the battery can be safely charged. The OVP control circuit 11 and the transistor switch M1 can be taken combinationally as an OVP circuit (as shown in the dotted line). In charging process, the charger control circuit 12 controls the gate voltage of the transistor M2 to determine the amount of the charging current.
The prior art shown in FIG. 1 requires two power transistors M1 and M2. Accordingly, it has the following drawbacks: first, more cost; second, the two large transistors connected in series in the charging path cause higher resistance, thus reducing charger efficiency and causing more severe heat dissipation issue.